What is a mask on a PCB?

Mask in this context means solder mask. An insulating paint that protects the traces from oxidation, dirt and shorts when soldering.

The bottom side shows you the raw material without the soldermask. The top side has white soldermask with black silkscreen (the text).

You are correct on the reason why cutting traces on these boards can be beneficial.

Now, regarding the mask: on regular PCBs, there is a layer of insulating "paint" that is silkscreened on the top of the boards after the copper is etched, mainly for the following reasons:

• prevent involuntary short-circuits if the board touches something conductive.
• protect the copper from dirt/contamination/oxidation.
• prevent solder bridge between adjacent pads during the reflow soldering process

It is usually green on most PCBs (this color comes from the mask, not the PCB core material). The mask goes all over the board surface, except where pads are, so that components can be soldered.

On this board, on the bottom side, they didn't put any mask at all (which is why it isn't green - or white, as the top side is), so that it doesn't annoy you when trying to cut traces. The mask could make it more difficult to reach the copper with an exacto knife.